At the beginning of an ingot cast, such as in an aluminum casting process, it is common in the first 300 mm of the cast for metal meniscus to contract and pull away from the mold on the short faces and corners. This phenomenon can occur for various reasons.
First, there can be inadequate metal flow into the corner and short face, which allows the metal to cool and pull away from the mold surface. Typically this inadequate flow is rectified by designing metal distribution systems which preferentially redistribute metal into these areas or by minimizing butt curl, which has in a roundabout way the tendency to restrict metal flow to the corner and short face.
Second, there can be excessive liquid molten-to-mold interface surface tension, which is typically an aspect of the alloy being cast. Alloys which can experience this problem include Aluminum alloys of Magnesium and/or Lithium. In some cases these alloys can be modified by surface active elements, such as, for example, Strontium, Calcium and Beryllium.
Third, there can be excessively tight corner radii. This problem can sometimes be resolved by using more liberal radii, but with a compromise of ingot scalping and hot line edge recovery. Generally, compromises made for start of the cast dynamics and recovery affect the total ingot recovery negatively in the hotline, where millions and millions of pounds are lost each year.
If such compromises are not made, overall ingot recovery is affected along with the inherent EHS aspect of metal dribbling into the mold to meniscus gap that can potentially create a butt hang-up, which can in turn cause a severe ingot explosion.
In some conventional processes, during curl, 150-250 mm into the cast, operators are continually on the casting table to make sure that the mold to meniscus gap is continually filled. From time to time they intervene and mechanically pull the metal control pin, or shake the pin-bag, to allow a sudden disruption to the metal level system to statically overcome the surface tension effect and “fill in” the corner or short face gap.